One expert says two minutes is "an eternity" on a flight — and the incident, on a Boeing 777-200 carrying 303 people, has sparked calls for a third crew member to be added to flight decks so no one is ever alone in the cockpit.

Air NZ spokeswoman Marie Hosking said the first officer and crew became concerned after the captain did not respond to three requests over two minutes from a cabin crew member to open the cockpit door.

The first officer eventually used an alternative method to access the cockpit. For security reasons, the airline would not say how.

"Naturally, cabin crew operating the flight were concerned about the inability to contact the captain and became quite anxious," said the national carrier's operational integrity and safety manager Errol Burtenshaw.

They were offered the support of the company's employee assistance programme after the flight.

Both pilots were stood down — the captain for two weeks and the first officer for a week, and given counselling and additional training.

There was "some tension" between the pilots after a 13-minute delay to the flight's departure after the first officer had to take part in a random drug and alcohol test.

"This departure delay frustrated the captain who prides himself on operational efficiency."

Safety and security were paramount and the incident was "unfortunate", Burtenshaw said.

"Both pilots have learned a valuable lesson around the need to communicate better with peers."

He said the captain did not respond or open the door because he was approaching a navigational waypoint and in his cockpit monitor saw a cabin crew member rather than the first officer ringing.

The airline provided a report on the incident to the Civil Aviation Authority. Spokesman Mike Richards said it was satisfied with Air NZ's actions.

But aviation commentator Peter Clark said the incident showed it was time all airlines put a third crew member in the cockpit. "After [the disappearance of Malaysia Airlines flight] MH370 there's definitely questions being asked about whether there should be more than two people on the flight deck."

The Beijing-bound Boeing 777 and its 239 passengers and crew vanished after the plane left Kuala Lumpur on March 8. A Malaysian investigation last month identified the captain as the chief suspect, if human intervention was to blame.

Clark said there was no excuse for the Air NZ captain to not immediately respond to calls, given the MH370 mystery and the fate of other flights, including an Ethiopian Airlines flight hijacked by its asylum-seeking co-pilot this year.

"You can push a button and say 'I'm busy' ... two minutes is an eternity when people reflect on MH370. The transponder can be turned off, the flight co-ordinates changed, the plane depressurised.